Leeds reached their 12th Challenge Cup semi-final in the past 14 years, overcoming a determined challenge from Harlequins with a compelling second half performance inspired by the introduction off the bench of the giant Kiwi forward, Ali Lauitiiti.
Holding a tenuous 12-10 lead and with the visitors in the ascendancy after quick-fire tries by Luke Dorn and Michael Worrincy just after half-time, Leeds suddenly came to life with a decisive four-try scoring burst in the space of 10 minutes.
Lauitiiti's offloading skills twice unlocked the Harlequins defence in the three decisive minutes that turned the tie. His one-handed pass enabled Chev Walker to put Scott Donald over before he combined with Donald to send Danny McGuire cutting inside. The teenaged winger Danny Williams and Donald added further tries with Leeds now firmly in control.
Though Pat Weisner and Tyrone Smith pulled back tries for Harlequins, Williams completed his hat-trick in the last minute, with an opportunist score from Rob Burrow's long raking kick. Leeds initially made heavy weather of the tie and their hospitalised coach Tony Smith, watching on television while recovering from a gastric illness, could hardly have felt much better as his side struggled for fluency. Assistant coach Brian McDermott took control in Smith's absence.
Harlequins created the best chances to break the deadlock, Solomon Haumono being held up over the home line in a massed tackle while Henry Paul, after following up his own clever chip kick, was just unable to grab the ball by the posts.
Leeds finally broke a tense deadlock after 32 minutes, with Willie Poching collecting Kevin Sinfield's pass and barging through Paul's tackle and past the covering Mark McLinden for the opening try. Six minutes later Burrow's elusive crossfield run set up Williams who evaded Nick Bradley-Qalilawa's challenge for a try again converted by Sinfield.
Leeds: L Smith; Donald, Walker, Senior, Danny Williams; McGuire, Burrow; Jones-Buchanan, Diskin, Scruton, Peacock, Ellis, Sinfield. Substitutes used: O'Neill, Millard, Lauitiiti, Poching.
Harlequins: McLinden; Bradley-Qalilawa, Luisi, Purdham, T Smith; Paul, Dorn; Tookey, Randall, Lolohea, Haumono, Hopkins, Mbu. Substitutes used: Danny Williams, Mills, Weisner, Worrincy.
Referee: K Kirkpatrick (Warrington).